The present invention relates generally to improvements in tube banks in heat exchangers, more particularly new means for the assembly or fixation of the tubes in a tube bank heat exchanger for longitudinal flow externally of the tubes of the kind provided with a plurality of mutually spaced support grates or the like with bars running through the free spacing between the tube racks.
The invention is developed in connection with heat exchangers for use in the chemical industry, for instance for petro-chemical processes wherein large heat quantities via correspondingly large gas quantities, frequently having relatively high pressure shall be interchanged with other gas volumes, high pressure feed water - steam or other fluids.
Heat exchangers in accordance with the invention can, however, also be utilized in connection with more conventional operations.
When for instance, heat is transferred from a gas quantity having lower or intermediate pressure to for instance high pressure steam or feed water, it is frequently known for productivity and economical reasons to convey the fluid having the higher pressure through U-shaped tubes. Thereby it will be sufficient to operate with one single tube plate and one cover for the high pressure. In order that the heat exchanger shall be able to operate on the basis of pure counter current principles, it is possible to arrange a baffle in the mantle, between each branch of the U. For assembling or fixation of the tubes there is then usually provided combined support- and conveying plates (baffles) which with regard to the support must consume at least one half of the cross-sectional area. The gas is then led partly longitudinally and partly transverse relative to the tube bank. The area available for gas flow and defining the gas velocity will in this manner then only amount to about 1/4 of the cross-sectional area of the mantle divided by the total cross-sectional area of the tubes (i.e. the open cross-sectional area between the tubes). In order to limit the velocity and pressure losses to acceptable values and simultaneously keeping the dimensions on high pressure parts within certain limits, it is frequently necessary to divide the assigned heat exchange task among several parallel units.
It is possible to let the gas flow in one direction only and obtain utilization of one half of the free cross-sectional area between the tubes. The counter current principle is then abandoned, a fact which with regard to the temperature difference may make it necessary to have two heat exchangers in series.
The gas flow transversely and longitudinally results further in relatively large pressure losses and also in great variations in velocities and leads to risk of vibrations. Such can also be formed due to the fact that the apertures in the baffles must be made with a certain clearance for the tubes.
Regarding the heat transfer number (coefficient) on the gas side, it is generally accepted that the combination of longitudinal and transverse flows which is obtained by means of baffles, gives a higher heat transfer number than pure longitudinal flow, provided the same Reynold number is prevalent. (A dimensionless number designating the heat transfer, the value of which depends upon the mass flow, characteristic dimensions (tube diameter) and viscosity).
With conventional calculation methods the above conclusion is correct in connection with heat exchangers subjected to low or intermediate load (Reynold numbers being less than approximately 20 - 30,000). At higher Reynold numbers these conditions will however, move towards a benefit when using pure longitudinal flow, such that at Reynold numbers of about 100,000 - 300,000 which frequently prevail in highly loaded heat exchangers as before mentioned, the heat transfer number on the gas side will be considerably higher at longitudinal flow. The calculations will then also be more correct since one can utilize the calculation methods applicable for fluid flow in tubes.
With regard to the condition that heat transfer conditions at large Reynold numbers are better at pure longitudinal flow than at combined transverse- and longitudinal flow, the inventors believe that the uneven velocities and the "dead zones" prevailing at the latter flow pattern will make a greater influence at higher Reynold numbers.